Dust diaries Tiny grains of dust are providing the earliest glimpse of the primordial solar system before Earth was born.

The particles, called calcium-aluminium rich inclusions or CAIs, are found inside meteorites. Scientists believe these 4.57 billion-year-old particles acted as tiny travel diaries, tracing their journey through the early solar nebula.

The discovery by a team of researchers including Dr Justin Simon, an astromaterials specialist with NASA's Johnson Space Center in Houston, is shedding new light on the processes that created the protoplanetary disk of matter around the early Sun.

Simon chose a pea-size CAI from the Allende meteorite, the largest carbonaceous chondrite ever found on Earth.

Estimated to have been the size of a car, it broke up as it fell through the atmosphere in 1969, showering the ground in Chihuahua, Mexico, with hundreds of pieces.

After cross-sectioning, they found distinct mineral layers had formed along the rim of the core, like layers of an onion.

Oxygen isotopes were the key

Simon used an ion microprobe called NanoSIMS (secondary ion mass spectrometer) to sample oxygen isotopes in micrometre-scale layers inside the CAI with resolutions of just 2-microns.

Because the relative abundance of oxygen isotopes varied in the protoplanetary disk, Simon and colleagues were able to pinpoint where a mineral formed.

Reporting in the journal Science, the researchers found the CAIs probably first condensed out of molecular gas into a solid, in an oxygen-16 rich environment near the Sun.

"It then encountered an oxygen poor environment, losing oxygen-16, possibly by being thrown out of the plane of the solar system, before falling into a planet-forming environment around where the asteroid belt is now," says Simon.

"I was surprised to find that it eventually recirculated back to the inner solar system."

But the findings, according to Simon, are consistent with some theories about the early protoplanetary nebula.

The X-wind theory

According to one theory the Sun's magnetic fields churned the gas and dust inside the early protoplanetary disk, tossing out grains formed near the Sun .

Once expelled, the grains fell like rain into the outer solar system and eventually incorporated into asteroids and planets.

According to Simon, the work follows on from decades of cosmochemistry and astrophysics research.

"Genesis defined the Sun's composition in terms of oxygen isotopes. That's where the oxygen-16 comes from," he says. "Stardust collected comet dust finding what looked like CAI's and providing the first evidence for a transfer of material from the inner to the outer solar system."